The E188CC is a special quality double triode. It is built to withstand operation in environments of shock and vibration, typically in aircraft and industry. Designed to operate under cut-off conditions for long periods it is also useful for computer logic circuits.
The design is aimed at RF, AF and cascode circuits at up to 200 MHz.
The second top mica is present to greatly reduce the effects of vibration. With a single mica, vibration opens the punched holes and the elements move and escape causing valve failure.
The working face of the anode is the small centre part. The outer lobes are removed from the electron stream to reduce capacitance but still offer good heat radiation.
Thanks to John Sykes for sending the photographs.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 47 mm tall.
Reference: Datasheet. Type E188CC was first introduced in 1959.